CGAL users discussion list

[Andreas Fabri <>]

Edge is a std::pair<Face_handle,int>

From the face you can get the two vertices
with index different from the index in the pair.

http://www.cgal.org/Manual/latest/doc_html/cgal_manual/TDS_2_ref/Concept_TriangulationDSFaceBase_2.html#Cross_link_anchor_1316

andreas


On 06/06/2011 13:27, Markus Eich wrote:
> Thank you for the input. My question is now how can I access the
> neighbors of an edge using CGAL?
>
> Is there an overridden function like as.segment(*it).get_neighbor() ?
> The link
>
> http://www.cgal.org/Manual/latest/doc_html/cgal_manual/TDS_2_ref/Concept_TriangulationDSFaceBase_2.html#Cross_link_anchor_1316
>
>
> doesn't tell me anything (sorry i am a CGA newbie). If I have the
> segment A from the Alpha_shape_edges_iterator, how can I get the
> adjactend Vertices. How can I extend my little example to get the two
> adjacend edges?
>
> ////////////////////
> for(Alpha_shape_edges_iterator it = as.alpha_shape_edges_begin();it !=
> as.alpha_shape_edges_end(); ++it) {
> segments.push_back(as.segment(*it));
>
>
>
> }
>
> ////////////////////
>
> Cheers,
>
> Markus
>
>
>
> On 06.06.2011 08:55, Sebastien Loriot (GeometryFactory) wrote:
> > Markus Eich wrote:
> > > Dear all,
> > >
> > > I am using CGAL in order the get one or more alpha polygons from a
> > > shape. IN a first approach I assume there are no holes. I generate
> > > the segements as follows:
> > >
> > >
> > > Alpha_shape_2 as(lp.begin(),lp.end());
> > >
> > > // find optimal alpha value
> > >
> > > Alpha_iterator opt = as.find_optimal_alpha(1);
> > > as.set_alpha((*opt)*2.0f);
> > > as.set_mode(Alpha_shape_2::REGULARIZED);
> > >
> > > std::vector<K::Segment_2> segments;
> > > std::vector<K::Segment_2> sorted_segments;
> > >
> > > for(Alpha_shape_edges_iterator it = as.alpha_shape_edges_begin();it
> > > != as.alpha_shape_edges_end(); ++it)
> > > segments.push_back(as.segment(*it));
> > >
> > > std::cout << segments.size() << " alpha shape edges" << std::endl;
> > >
> > >
> > > This works fine an I get all the segments. My problem is that I need
> > > a single polygon (in my example I only have one concave shape). If I
> > > iterate over the std::vector<K::Segment_2> segments vector, the
> > > segments are like E,A,V,D,C,F , instead of A,B,C,D,E,F. In need the
> > > segments in an ordered way i.e. that the .target() of A is the
> > > .source() of B, the .target() of B the source of C, and so on.
> > >
> > > How can I achieve this using CGAL? Is there a way to sort the
> > > segments directly?
> > There is nothing to do it directly in CGAL.
> >
> > This is a simple graph problem: each edge has 2 vertices and two
> > adjacent edges share one identical vertex. In particular each such
> > vertex is adjacent to two other vertices. Starting from one vertex
> > and going from adjacent vertex to adjacent vertex you can reconstruct
> > the polygon.
> >
> > You can use for example a
> > std::map<Vertex_handle,std::vector<Vertex_handle> >
> > to get the adjacency between vertices.
> >
> > For the definition of an edge as std::pair<Face_handle,int>, see:
> > http://www.cgal.org/Manual/latest/doc_html/cgal_manual/TDS_2_ref/Concept_TriangulationDSFaceBase_2.html#Cross_link_anchor_1316
> >
> >
> >
> >
> >
> > S.
> >
> >
> >
> >
> > > Thank you and cheers,
> > >
> > > Markus


--
Andreas Fabri, PhD
Chief Officer, GeometryFactory
Editor, The CGAL Project

phone: +33.492.954.912    skype: andreas.fabri